1. Field of the Invention
The present invention relates to reactive cords and connectors therefor and, more particularly, to cords which are sufficiently rigid for insertion through a material to be fractured by the cords.
2. Related Art
Detonating cords are well known and typically include a core explosive material covered by a non-metal sheath. The sheath may comprise an extruded flexible plastic inner jacket and a flexible, textile outer jacket composed of, for example, polyester yarn. The detonating cord sheath may also include a waterproofing and sealing material such as asphalt disposed about the core. The core explosive may be composed of, for example, pentaerythritol trinitrate (PETN), cyclonite (RDX), homocyclonite (HMX), tetranitrocarbazol (TNC), hexanitrostilbene (HNS), 2-6-bis picryo-amino 3,5-dinitro pyridine (PYX) or black powder, typically with a plasticizer such as a polysulfide and/or one or more other known additives. A typical core loading of PETN may be on the order of 7.5 to 50 grains per foot (gr/ft) (about 1.6 to 10.6 grams per meter (g/m)) with a detonation velocity of about 21,000 feet per second (about 6400 meters per second) or about 4 miles a second (about 6.4 kilometers per second). Detonating cords are typically used in the initiation of charges of high explosives but have also found other applications, including the removal of combustion residues formed on boiler tubes in steam generation plants as described below. A cross-sectional view of a typical prior art detonating cord 30xe2x80x2 produced by the Assignee of this application is shown in FIG. 1A and comprises a core 38 of explosive material, about which a multi-layer, non-metal sheath is disposed. The sheath comprises a thin plastic containment jacket 35 which contains the core material and two layers 31 and 33 of textile casings. The Assignee also produces a detonating cord under the trademark PD CORD. The product is an all-purpose detonating cord comprising an explosive core encased in a textile, which in turn is covered with a plastic jacket. These products remain flexible enough to allow knot tying and spooling of lengths measuring hundreds of feet onto a three-inch diameter spool. A more rigid detonating cord produced by the Assignee is identified by the commercial designation PRIMACORD 400, whose stiffness is a result of its high core load (400 gr/ft) and diameter (about 0.5 inch). Even this product is sufficiently flexible to be wound onto six-inch spools.
Detonating cord as is known in the art is so flexible that it can be tied in knots with other flexible cords for purposes of detonation signal transfer from one to another. The high degree of flexibility of known detonating cord makes it necessary to either lay the cord where desired or pull it into position since it lacks sufficient rigidity to be pushed into place. Like-wise, detonating cord cannot easily be pushed through a small passageway, especially if the passageway is irregular or has bends or kinks, and it cannot be pushed so as to penetrate fly ash or another soft substance for any significant distance.
Steam generation plants generate steam for various uses, e.g., to drive turbines for the generation of electricity or to provide steam to heat large buildings. Such plants typically combust a fuel, e.g., coal, to heat a bank of water-containing boiler tubes to generate the steam. One side product of the combustion is air-borne fly ash, which is typically a mixture of alumina, silica, carbon, hydrocarbons and various metallic oxides. Over time, fly ash, along with other particulates such as dust, builds up and solidifies on the surface of the boiler tubes and may even fill the spaces between the boiler tubes. The fly ash and other residues vary considerably in density from a powdery consistency to a cement-like scale. When such residues cover the boiler tubes, they thermally insulate the tubes from the flames used to heat them and thereby reduce the efficiency of heat transfer and thus the efficiency of the boiler. Accordingly, from time to time, the caked fly ash and other residues must be removed from the banks of boiler tubes in order to return the efficiency of the steam plant to acceptable levels.
Removal of the caked fly ash from a bank of steam or boiler tubes is conventionally carried out by teams of workers, at least one team member standing or crouching on top of the bank of boiler tubes and another team member standing or crouching out of sight under the bank of boiler tubes, which is typically about several feet deep. The work process involves passing a detonating cord through the caked fly ash and around the tubes, and then initiating the detonating cord so that the fly ash and scale are broken up and are dislodged from the tubes. If the fly ash and/or scale leaves sufficient space between the tubes, it may be feasible simply to drop the detonating cord downward between the tubes. However, if the fly ash fills the spaces between the tubes or if the path between the tubes is narrow or irregular because of the fly ash, passages must be created in the caked fly ash to accommodate the detonating cord, which lacks sufficient rigidity to be pushed through the fly ash or to be guided and forced through a narrow or irregular path from above. The process of creating the passages is termed xe2x80x9croddingxe2x80x9d and involves the use of, for example, a bar and/or a saw forced between the boiler tubes by hand to create passages through the caked fly ash to receive the detonating cord. The bar and/or saw used is typically about 4 to 6 feet (about 1.2 m to 1.8 m) long in order to cut a passage completely through the caked fly ash on a bank of boiler tubes. This work is physically demanding and is often done in very confined spaces as the distance between banks of boiler tubes within a typical boiler may be as little as about 4 feet (about 1.2 meters). Moreover, many passages must be created as the detonating cord is usually wrapped with adjacent turns spaced apart by a distance of only about 12 to 18 inches (about 30 to 45 cm).
Once the passages have been bored or cut in the caked fly ash, detonating cord may be wrapped about the boiler tubes. First, the detonating cord end is dropped between the tubes from an upper level to workers on a lower level. The detonating cord may either pass through space left by the fly ash between the tubes or through a hole rodded through the fly ash. Thereafter, the detonating cord end is pulled back up to the upper level using a tool, for example, a rod with a hoop on the end. The detonating cord is connected to the hoop and the rod is used to thread the detonating cord through a passage formed in the caked fly ash. After the slack is taken in, the process must be repeated many times. Should the downward path be too irregular, too narrow or too obstructed by fly ash, it may be necessary to thread the flexible detonating cord downwards through the bank of boiler tubes as well as upwards. Finally, the detonating cord is detonated to fracture the scale and fly ash and permit their removal from the tubes. It will be appreciated that the foregoing is a laborious and time-consuming operation resulting in significant downtime for the boiler and significant labor costs.
U.S. Pat. No. 5,056,587, issued to Jones et al, on Oct. 15, 1991 and entitled xe2x80x9cMethod For Deslagging a Boilerxe2x80x9d, discloses the rodding technique described above. FIG. 3B shows a cross-sectional view of a horizontal tubing array having a plurality of tubing panels with explosive detonating cord wrapped around the tubes. Detonation of the cords separates the ash from the tubing panels. As taught at column 8, lines 12-14 and 33-38, the detonating cords used are known flexible detonating cords requiring rodding and/or threading, using tools as discussed above, and are wrapped tightly about the banks of tubes.
U.S. Pat. No. 5,211,135, issued to Correia et al, on May 18, 1993 and entitled xe2x80x9cApparatus And Method Of Deslagging A Boiler With An Explosive Blastwave and Kinetic Energyxe2x80x9d, shows the use of highly flexible detonating cords in known methods of explosive deslagging. As seen in FIG. 1, bank 10 of boiler tubing panels 12 includes a plurality of spaced-apart links of boiler tube 14 held in place by spacer 16 (FIG. 4). The individual tubes 14 and panels 12 may be forty feet long. The boiler may comprise three hundred sets of tubing panels 12. Personnel referred to as xe2x80x9cblastersxe2x80x9d hand fashion a series of loops 20 of detonating cord (FIG. 2) into loop clusters 22 which are disposed between the tubing panels to provide explosive assemblies 28. This illustrates the very high flexibility of known detonating cord.
Atlas Corporation distributed in the United States special low-density explosives for pre-splitting and smooth blasting operations under the trade name KLEEN KUT(trademark). The explosives were in 36-inch long cartridges which could be rigidly interconnected by couplers. The cartridges were offered with a minimum of 0.19 pounds of explosive per foot (about 1330 grains per foot) for use in pre-splitting, slope control, cushion blasting and smooth blasting and were manufactured with a special cartridge wrap to facilitate underground use.
The Assignee of this application also produces lead-sheathed detonating cords under the trademarks PRIMACLAD and PRIMASTICK. The lead sheath provides protection from hostile environments such as high temperatures encountered in oil field work. Lead, however, does not provide resiliency to the detonating cord and has additional disadvantages in certain applications. Metal-clad detonating cords are manufactured by filling a metal tube with explosive material and then subjecting the tube to a plurality of drawing (lengthening) steps. The process inherently involves the addition of substantial energy to the product, which increases the danger of manufacture. The finished metal-clad detonating cords are more difficult to initiate than plastic- and fabric jacketed cords. Initiation of metal clad detonating cords requires either higher output detonating cord, a special donor or special connectors to attach a donor cord across an exposed cut end of the metal clad cord. Furthermore, lead and other metal sheathings are extremely disadvantageous for use in cleaning of boiler tubes. Upon detonation, the metal may form shrapnel that can damage the surrounding structures, including the boiler tubes, which may suffer points of direct structural weakness or hot spots, resulting in long term degradation of the boiler tubes. Furthermore, substantial portions of the metal sheath may be vaporized and deposited on the tubes, again causing structural weaknesses or hot spots. Further still, the lead will adversely affect catalytic converters in the boiler exhaust stream and adversely impact the local environment. Finally, the ash collected from the tube cleaning process is customarily sold for ceramic use and metal contamination is undesirable. Thus, numerous disadvantages are known to arise from the use of the lead-sheathed detonating cord for deslagging a boiler, making their use unacceptable.
The present invention provides an improved reactive cord comprising a core of reactive material and a non-metal sheath produced using a continuous extrusion process surrounding the core, the improvement comprising that a six-foot length of the cord is sufficiently rigid to perforate fly ash.
The present invention also provides a reactive cord wherein the sheath comprises a material having a flexural modulus of about 250,000 psi (17.236xc3x97102 MPa).
In another aspect, the present invention provides a cord comprising a core of reactive material and a non-metal sheath produced using a continuous extrusion process surrounding the core, wherein the cord is sufficiently rigid so that, when a six-foot length is supported horizontally at one end, the opposite end dips not more than about twelve inches from horizontal.
According to another aspect of the invention, the cord may comprise explosive material with a loading of less than 5000 grains per foot, optionally less than 1000 grains per foot, further optionally less than 500 grains per foot.
Optionally, the sheath of the cord may comprise an extruded jacket comprising one or more selected materials from the group consisting of: polystyrene, polycarbonate, polyamide, polyamide-imide copolymer, ethylene-chlorotrifluoroethylene copolymer (ECTFE) and acrylonitrile-butadiene-styrene (ABS) copolymer.
The sheath may comprise at least two layers including an innermost layer comprising a sealant jacket in contact with the reactive material and an outer jacket layer, which may comprise a plurality of longitudinally disposed reinforcing fibers.
According to one aspect of the invention, one end of the rigid cord is configured in the shape of a hook and comprises a shank, a return bend and a tip. This embodiment may be combined with a connector comprising a first hole and a second hole dimensioned and configured to receive the shank and the tip of the cord therethrough.
According to another aspect of the invention, the sheath may optionally have a noncircular cross-sectional configuration. Alternatively, it may have a wagon-wheel cross-sectional configuration.
This invention also has method aspects, such as a method of installing a reactive cord within a bank of boiler tubes caked with fly ash, comprising pushing the cord between the tubes to position the cord in the fly ash. In some instances pushing the cord may comprise perforating the fly ash with the cord. In other instances pushing may comprise pushing the cord upward through the tubes.
There is also a method of removing caked fly ash from a bank of tubes, comprising pushing a plurality of rigid reactive cords between the tubes to position the cords in the fly ash, and initiating the rigid cords. This method may comprise arranging a donor line in signal transfer relation to the rigid cords and initiating the donor line.
Further, there is a method for producing a rigid reactive cord comprising depositing a non-metal jacket over a flexible reactive cord which comprises a core of reactive material and a non-metal sheath. This method comprises depositing at least one additional non-metal jacket layer over the flexible cord in a continuous extrusion process to produce a cord having the rigidity described herein. Optionally, the additional jacket layer comprises a high modulus material. The method may comprise extruding the jacket over the flexible cord and cutting lengths of the jacketed cord during the extrusion process. The method may optionally comprise depositing a plurality of reinforcing fibers with the additional jacket layer.
Other methods of this invention include pushing a length of rigid detonating cord into a column of explosive material or into a bore hole.